As is pointed out in U.S. Pat. No. 4,110,400, the recovery of nickel and cobalt from acidic leach solutions by precipitation as sulfides using hydrogen sulfide under pressure has been applied in the Moa Bay plant in Cuba. The process has been studied at length and much progress has been made in terms of overall recovery of nickel and cobalt, economy in energy and reactant requirements, sulfide product granulometry and purity, production rate, etc. The said U.S. Pat. No. 4,110,400 constituted a major advance since the process there disclosed permits use of hydrogen sulfide at relatively low pressures and temperatures while maintaining high production rates. Greater safety, simplification of equipment, reductions in internal scaling, corrosion and maintenance have resulted.
Briefly, the process described in U.S. Pat. No. 4,110,400 contemplates production of nickel sulfide by sulfide precipitation from an acidic solution containing up to 40 gpl nickel, e.g., up to 15 gpl nickel, about 30 to about 400 gpl of a soluble inert sulfate such as magnesium sulfate, and a pH of about 1.5 to about 4. The soluble inert sulfate acts as an acid buffer to combine with hydrogen ions formed during the precipitation of nickel so as to form HSO.sub.4 ion. Finely divided nickel sulfide in the amount of at least 75 gpl of nickel-containing solution is used as seed during nickel sulfide precipitation from the solution using hydrogen sulfide under pressure of up to 50 psig and temperature of 65.degree. to 100.degree. C. Provision for introduction of MgO slurry for control purposes was made. Many advantages result from the process described in U.S. Pat. No. 4,110,400, but it is found that when the sulfide precipitation reactor or autoclave is operated on a continuous basis for an extended time period that the nickel sulfide production rate decreases to an unacceptable extent. It is to the solution of this problem that the present invention is directed.